This invention relates to a rolled film capacitor and, more particularly, to a capacitor formed by winding into a roll a pair of dielectric film strips such as paper strip or plastic film strip each having metallic electrodes deposited thereon.
An example of this type of capacitor is disclosed in the Japanese patent opening gazette No. H1-162318. In the capacitor of the cited reference, a suitable metal such as aluminium is evaporated onto a film strip of dielectric material such as polyethylene terephthalate to form a number of rectangular island electrode regions which are regularly arrayed in lateral and longitudinal directions. A pair of such strips are piled one upon another to form a number of capacitor elements and the strips are then laterally shifted one from another to serially connect the capacitor elements in the lateral direction for raising the breakdown voltage. The mutually overlapping strips are then wound into a roll to form a unit capacitor. A plurality of such unit capacitors are connected in series and/or parallel to form a capacitive power supply having necessary breakdown voltage and electrostatic capacitance.
Recent power supply devices of this kind are generally large in size and it has naturally been requested to reduce this size. Most of the volume of such unit capacitor is occupied by the dielectric material and, as well known in the art, a cumulative energy J.sub.c of such dielectric is given by an equation, J.sub.c =.epsilon.E.sup.2 V/2, where .epsilon. is a dielectric constant, E is a field strength and V is the volume. Therefore, the cumulative energy per unit volume is proportional to square potential gradient. In other words, the most effective measure for reducing the size of capacitor is to increase the potential gradient of the dielectric.
Polyethylene terephthalate films and polypropylene films have generally been used as the dielectric material having high breakdown voltage. Although the breakdown voltage of these films is as high as 400 to 600 killovolts per millimeter, breakdown may happen when it is used in a capacitor and it has a large area and includes some partial defects, unless the design is effected with an allowable voltage lower than the lowest value of this breakdown voltage.
When a capacitor bank having large capacitance and large cumulative energy (e.g., one megajoule) is constructed, a number of unit capacitors are connected in parallel. If breakdown happens in one of the unit capacitors, the energies accumulated in the unit capacitors connected in parallel with it are concentrically discharged to the unit capacitor having broken down to cause an explosive destruction in danger of scattering its fragments and causing a fire.
In order to prevent these accidents, the unit capacitors have been designed with the potential gradient of the dielectric between 70 and 150 kilovolts per millimeter, which is lower than the actual breakdown voltage of the dielectric films by a factor of three to eight. This is the greatest cause of the large size of the capacitors.
Accordingly, an object of this invention is to remove from the capacitor function the fine defects regarding the breakdown voltage which are dispersed on the dielectric film and to construct the capacitor only with those parts of the film having ideal breakdown voltage, thereby increasing the potential gradient for use and reducing the size of the capacitors.
Another object of this invention is to significantly improve insulation recovering feature of the capacitors after breakdown in a defective portion of the film.
The above objects can be attained by a rolled film capacitor provided in accordance with this invention.